Various semiconductor light emitting elements have been developed such as light emitting diode and laser diode. These semiconductor light emitting elements are replacing part of electric bulbs and cold cathode ray tubes as light sources for display, back light and indicator, by taking advantage of such features as the capability to run on low voltage, small size, light weight, small thickness, long life, high reliability and low power consumption.
In recent years, such light emitting devices have been developed as nitride semiconductor is used as the light emitting element that can efficiently emit light in a region raging from ultraviolet to visible light of short wavelengths. Such LEDs capable of emitting blue or green light with 10 candela or higher luminous intensity have been commercialized by using light emitting elements having quantum well structure with activation (light emission) layer made of nitride semiconductor (for example, mixed crystal of InGaN).
It has also been made possible to carry out color mixing including white color by combining the light emitted by such an LED chip (light emitting element) and the light emitted by a fluorescent substance that is excited by the light of the LED chip thereby to produce fluorescence.
Such light emitting diodes are disclosed, for example, in Japanese Non-examined Patent Publication No. 152609/1993, Japanese Non-examined Patent Publication No. 153645/1997 and Japanese Non-examined Patent Publication No. 10-242513/1998.
Specifically, the light emitting element is caused to emit ultraviolet rays or blue light of relatively short wavelengths in the visible region, and the fluorescent substance is excited by the light of this light emitting element so as to emit visible light of wavelengths longer than the light emitted by the light emitting element. The constitution that employs color mixing of the light emitted by the light emitting element (visible light) and the light emitted by the fluorescent substance has an advantage that the structure can be simplified and the output power can be increased, although color drift is likely to occur. When a light emitting element that emits ultraviolet rays is used, on the other hand, such a constitution is employed as fluorescent substances capable of emitting light of red, green and blue colors so as to produce white light are used, so that the color can be adjusted relatively easily since only the light emitted by the fluorescent substances is used. When a light emitting element that emits ultraviolet ray is used, in particular, mass productivity can be improved since deviation of wavelength of the light emitted by the semiconductor light emitting element can be accommodated somewhat and the chromaticity can be determined only by the color of light emitted by the fluorescent substance.
However, in the light emitting device of the prior art where two or more kinds of fluorescent substance are used and a desired color is produced by color mixing of the light emitted by the fluorescent substances, intensity ratio of light emitted by two fluorescent substances changes due to difference in the excitation spectrum between the two fluorescent substances, when emission wavelength of the light emitting element used in excitation changes.
As a result, in the light emitting device of the prior art where two or more kinds of fluorescent substance are used, it has been difficult to satisfactorily suppress the variation of chromaticity of the light emitting device since variability in the emission wavelength of light emitting element affects the chromaticity of the light emitting device.
For the semiconductor light emitting element to be used as light source including illumination by making use of the advantage thereof, the light emitting device of the prior art is not satisfactory and it is required to increase the luminance and improve the mass productivity.
There has not been known any fluorescent substance that can be excited by near ultraviolet rays or visible light of short wavelengths so as to emit red light with sufficient luminance.
Therefore, there is such a case as the proportion of mixing the red light emitting fluorescent substance must be increased, which results in a decrease in the relative luminance. When the fluorescent substance is excited by near ultraviolet rays, variability in the wavelength of light emitted by the light emitting element may lead to a decrease in the luminance of red light emitted by the fluorescent substance due to the shift in the wavelength of light that excites the fluorescent substance, thus resulting in a change in chromaticity of light emitted by the light emitting device.
Also in case the fluorescent substance is excited by a fluorescent substance emitted by the light emitting element, it is desired that a required color can be achieved by controlling the composition of the fluorescent substance in order to improve the mass productivity.